Actually lost mileage in my Gf's car. A couple of things happened too.
1: Oil from the filter built up on the intake air temp sensor causing incorrect data sent to the computer
2: Throttle bore and throttle plate got dirtier faster (dirt sticks to oil from filter)
3: Reduced fuel economy... my theory belowAs more air is introduced into the motor, the ECU (engine control unit) by way of the oxygen sensors, will sense a leaner air/fuel ratio. It will then respond by adding more fuel (keeping fuel injectors open longer) to maintain it's desired air/fuel ratio. Power will slightly increase and should only manifest itself at WOT (wide open throttle).

Edit: Most washable air filters will allow more air into your engine, but as a trade off it will also allow more dirt to get in as well. More dirt into your engine = reduced engine life

If you get a complete INTAKE system, then maybe; but just replacing your paper air filter with an K&N (or whatever) will most likely will not.

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This is actually a true statement. If you dont change the resitrictive nature of the cars Intake system with the use of Baffles fopr sound reduction and such, you dont get any inprovement with the airfilter itself, And then even with the entire system, you dont see a Major Improvement untill you flash the PCM to match the now added amount of air, until you do so the vehicle, fuel injection anyways, will run lean, which in turn will raise cylinder temps, raise NOx emissions, and wear your engine faster.

Not much difference in the gas mileage. They will give you more power on the top end, but they need to be washed and oiled monthly, which is a pain in the rear. If you don't, you will be getting dirt in your motor.

I tried one on my 2005 Dodge Dakota V8. Initially, it increased power (slightly) but didn't do a thing for gas mileage. As the filter got dirty, which it did rather quickly, the performance and mileage statred dropping steadily. The cleaning procedure is such a pain in the rear that I gave up on K & M and went back to a stock filter. I'd rather find a high quality paper filter.